A fuel cell has been proposed as a clean, efficient and environmentally responsible power source for various applications. In particular, individual fuel cells can be stacked together in series to form a fuel cell stack capable of supplying a quantity of electricity sufficient to power an electric vehicle. The fuel cell stack has been identified as a potential alternative for a traditional internal-combustion engine used in modern vehicles.
Fuel cells are electrochemical devices which combine a fuel such as hydrogen and an oxidant such as oxygen to produce electricity. The oxygen is typically supplied by an air stream. The hydrogen and oxygen combine to result in the formation of water. Other fuels can be used such as natural gas, methanol, gasoline, and coal-derived synthetic fuels, for example.
One type of fuel cell is known as a proton exchange membrane (PEM) fuel cell. The PEM fuel cell typically includes three basic components: a cathode, an anode, and an electrolyte membrane. The cathode and anode typically include a finely divided catalyst, such as platinum, supported on carbon particles and mixed with an ionomer. The electrolyte membrane is sandwiched between the cathode and the anode to form a membrane-electrolyte-assembly (MEA).
In a typical PEM-type fuel cell, the MEA is sandwiched between diffusion media or diffusion layers that are formed from a resilient, conductive, and gas permeable material such as carbon fabric or paper. In certain designs, the cathode and the anode are also formed on the diffusion media and sandwich the electrolyte membrane. The diffusion media serve as current collectors for the anode and cathode as well as provide mechanical support for the MEA. The diffusion media and MEA are pressed between a pair of electronically conductive bipolar plates which also serve as current collectors for collecting the current from the electrochemical fuel cell reaction.
The bipolar plate typically includes two thin, facing metal unipolar plates. One of the metal unipolar plates is an anode plate that defines a flow path on one outer surface thereof for delivery of hydrogen reactant to the anode of the MEA. An outer surface of the other unipolar plate, known as a cathode plate, defines a flow path for the oxidant reactant for delivery to the cathode side of the MEA. When the unipolar plates are joined, the joined surfaces define a path for a coolant fluid to flow therethrough.
The unipolar plates are typically produced from a formable metal that provides suitable strength, electrical conductivity, and corrosion resistance. In particular, stamped thin metallic sheets are typically used for the anode and cathode unipolar plates when forming the bipolar plate. For example, the metallic sheet may be less than about 100 micron in thickness. However, the thin material and high strength of the stainless steel material cause significant lateral (in-plane) spring back. It has been shown that the spring back can reach over 400-500 micron. The spring back causes an uneven distribution of a flow field stamped in the plate assembly, undesirably affecting a functionality of the resulting bipolar plate and fuel cell stack.
As shown in FIG. 1, the large spring back in the bipolar plate 100 may far exceed typical tolerances. The large spring back, if not sufficiently compensated for in a tool used to form the bipolar plate 100, may cause an interference 102 between cathode and anode unipolar plates 104, 106 during assembly of the bipolar plate 100.
To compensate for the lateral bipolar plate spring back, a method of using either global or local morphing to design and cut dies for stamping the bipolar can be used. However, for relatively large spring back (compared to feature size and design tolerance), for example, over 200 microns, the one step methods can undesirably cause severe surface distortion when trying to achieve necessary fidelity due to the large surface deformation.
There is a continuing need for a method providing stable and reliable surface compensation with high accuracy and surface quality for large lateral spring back of fuel cell bipolar plates, to consistently meet the requirements of manufacturing and assembly.